1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to an apparatus for controlling clearance between adjacent rotating and non-rotating components of a gas turbine engine.
2. Description of the Prior Art
The efficiency of a gas turbine engine is dependent upon many factors, one of which is the radial clearance between adjacent rotating and non-rotating components, such as, the rotor blade tips and the casing shroud surrounding the outer tips of the rotor blades. If the clearance is too great, an unacceptable degree of gas leakage will occur with a resultant loss in efficiency. If the clearance is too little, there is a risk that under certain conditions contact will occur between the components.
The potential for contact occurring is particularly acute when the engine rotational speed is changing, either increasing or decreasing, since temperature differentials across the engine frequently result in the rotating and non-rotating components radially expanding and contracting at different rates. For instance, upon engine accelerations, thermal growth of the rotor typically lags behind that of the casing. During steady-state operation, the growth of the casing ordinarily matches more closely that of the rotor. Upon engine decelerations, the casing contracts more rapidly than the rotor.
Control mechanisms, usually mechanically or thermally actuated, have been proposed in the prior art to maintain blade tip clearance substantially constant. However, these prior art control mechanism have drawbacks. Mechanical mechanisms typically are complex and, although capable of responding quickly, are subject to vibration and mechanical tolerance "stack-ups". Thermal mechanisms typically require thermally radially moving structural casings by air impingement on the casings and thus require thermal brute force to radially move structural casing shells and flanges/rings over an inordinately long period of time, such as up to three minutes. Thus, none of the prior art control mechanisms are believed to represent the optimum design for controlling clearance.
Consequently, a need still remains for an improved mechanism for clearance control that will improve engine performance and reduce fuel consumption.